1. Field of the Invention
The present invention is related to a method of measuring the degree of alloying of a galvanized steel sheet in the non-contact type by using laser beams. In more detail, the present invention is related to a method of measuring the degree of alloying by using laser beams by which precise measurement of the degree of alloying is enabled without errors coming from surface roughness and vibration of the galvanized steel sheet by using the photodiode arrays when measuring the intensity of the laser beam reflected from the galvanized steel sheet.
2. Description of the Prior Art
Galvanized steel sheets have been widely used for constructional materials, structural articles, and other objects. Particularly, the melt zinc galvanized steel sheet which is alloyed through the heat treatment process has superior anti-corrosiveness, weldability, coating property, etc., and thus demands for its application to home appliances and industry have been increased.
However, since such superior characteristics of the alloyed melt zinc galvanized steel sheet are greatly affected by the degree of alloying of the galvanized layer, i.e., the content of Fe component in the galvanized layer, it is necessary to manage the degree of alloying consistently for securing the quality of the produced steel sheet such that a proper degree of alloying is maintained suitably for its application objects.
Particularly, in these days wherein mass-production is common, it is necessary to control the alloying process in real time so as to prevent manufacture of inferior products and ensure superior quality of them so that it is essential to carry out an on-line measurement for the degree of alloying.
As one of the methods of measuring the degree of alloying of the galvanized steel sheet, a method using x-ray diffraction has been known, and is disclosed in Japanese Patent Application Publication (No. hei 5-45305). In this method, the degree of alloying is measured by detecting the intensity of diffraction differentiated from a melt zinc galvanized steel sheet, in which differentiated diffraction has a certain angle depending on the phases of Fe and Zn which are formed as the melt zinc galvanized layer is alloyed. In this method, however, the measuring angle and position of a photodiode for detecting the intensity of diffraction affect most greatly the accuracy of the results of the measurement. Therefore, there may be a problem that even if the location of a measuring object is changed slightly, the results of the measurement differ greatly.
That is, if the above-described measurement method is supplied to actual work, it is very likely that measurement errors occur due to the vibration generated when the steel sheet is moved over roll. Moreover, since the above-described method employs x-ray, there are problems that it is difficult to apply the method to the case of working at a high temperature, and safety measures have to be considered accordingly.
Further, as another method for measuring the degree of alloying of a galvanized steel sheet, there is a method using the characteristics of the melt zinc galvanized steel sheet in that its color is changed according to the degree of alloying, which is disclosed in Japanese Patent Publication (No. hei 4-370709). The method presented in the above publication is as follows: first, photograph the enlarged picture of the surface of the steel galvanized layer,, and compare the average brightness of the enlarged picture with the degree of alloying of the galvanized layer repeatedly. And then measure the degree of alloying by preparing a comparison chart between the average brightness of the enlarged picture and the degree of alloying of the galvanized layer. That is, when measuring the degree of alloying of an unknown galvanized steel sheet in this method, the average brightness is calculated by photographing the enlarged picture of the surface of the steel sheet, and then the degree of alloying is determined by comparing the average brightness thus obtained with the value of the comparison chart which has been made previously.
This method is advantageous in that this method is less affected by the vibration compared to measuring the degree of alloying by x-ray diffraction, but still has a problem that there may occur errors in the measurement according to the surrounding illumination state and surface roughness of the galvanized layer.
Accordingly, the present inventors have conducted researches on and suggested a method of measuring the degree of alloying by using laser beams which are applicable properly to the case of working at a high temperature, enable safer measurement of the degree of alloying, and is less affected by the surrounding illumination state and the change in the surface roughness of the galvanized layer. The results of their study are summarized in Korean Patent Application No. 96-44522, which is illustrated below with reference to FIG. 1.
In this method, firstly, a standard sample is installed inside of a measuring instrument (100), and the intensity of specular reflection I.sub.0 (.alpha.) and that of scattering I.sub.0 (.beta.) are detected. The basic degree of alloying (X.sub.0) of the standard sample is obtained by substituting the intensity of specular reflection I.sub.0 (.alpha.) and that of scattering I.sub.0 (.beta.) into Equation (1): ##EQU2## The intensity of specular reflection I.sub.1 (.alpha.) and that of scattering I.sub.1 (.beta.) of the standard sample (113) are detected, and the comparative degree of alloying (X.sub.1) of the standard sample is obtained by substituting the intensity of specular reflection I.sub.1 (.alpha.) and that of scattering I.sub.1 (.beta.) thus detected into Equation (1).
The arrangement of a laser generator (101), a first beam splitter (102), and a mirror (104) is corrected by comparing the basic degree of alloying (X.sub.0) and the comparative degree of alloying (X.sub.1) of the standard sample thus obtained. Then the degree of alloying (X.sub.2) of the galvanized steel sheet (109) is obtained by detecting the intensity of specular reflection I.sub.2 (.alpha.) and that of scattering I.sub.2 (.beta.) of the galvanized steel sheet (109) and substituting I.sub.2 (.alpha.) and I.sub.2 (.beta.) thus detected into Equation (1). In the meantime, reference numerals 103, 105, 106, 112, and 113 in FIG. 1 not illustrated here show photodetectors.
However, the above-described method of measuring the degree of alloying by using laser beams is problematic in that it is difficult to measure the degree of alloying precisely if the galvanized steel sheet, which is an object of measurement, is inclined by external vibration.
Also, during the on-line measurement of the degree of alloying of a galvanized steel sheet by using laser beams in the production line, the measurement is made while moving the measuring instrument in the widthwise direction of the galvanized steel sheet in order to obtain information on the degree of alloying as much as possible. When desiring to obtain information on the degree of alloying in the widthwise direction of the galvanized steel sheet by moving the measuring instrument, the width of the galvanized steel sheet has to be known accurately. In the past, it was measured directly by the worker. However, this method has been problematic in that not only is it difficult to measure the width of the galvanized steel sheet during the production work, but also it is possible to have errors in measuring the widthwise degree of alloying if the width of the galvanized steel sheet is changed. Further, in this method, there are problems that if the width of the galvanized steel sheet is changed in its production line, the operation of the measuring instrument has to be stopped and re-started after inputting new information on the width of the galvanized steel sheet.